Distributed optical fiber sensor with controlled response

ABSTRACT

Distributed fiber optic chemical and physical sensors can provide a relatively highly uniform response over the length of the fiber by, for example, varying such properties as the core/cladding index of refraction ratio to compensate for the non-linearity in sensitivity. The phenomenon of spatial transient over a length of a fiber introduces such a nonlinearity that can be compensated for by varying at least one parameter of the optical fiber.

RELATED APPLICATIONS AND PATENTS

This application is a continuation of application Ser. No. 11/364,237filed on Feb. 28, 2006 now U.S. Pat. No. 7,260,283, which is acontinuation of application Ser. No. 10/717,080 filed on Nov. 19, 2003now U.S. Pat. No. 7,006,718, which is a continuation of application Ser.No. 09/730,158 filed on Dec. 5, 2002 abandoned which is a continuationof application Ser. No. 09/334,845 filed on Jun. 16, 1999 now U.S. Pat.No. 6,205,263. The entire content of each of the above referredapplication is incorporated herein by reference.

The invention was made which Government support under contract NoNASA1-19895 and No NASA1-20206. The Government has certain rights in theinvention.

FIELD OF INVENTION

This invention relates to optical fiber sensors and more particularly tosuch sensors which include materials in the coatings and/or claddings ofoptical fibers which produce changes in the characteristics of the lightcarried by the fiber responsive to the presence of a material or fieldto be detected.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 4,7834,496 issues May 30, 1989 discloses distributed fiberoptic chemical sensors. Such a sensor comprises an optical fiber with acore and a cladding which is permeable. The cladding, or a coating onthe cladding, includes a composition, the optical properties of whichare altered in the presence of a material to be detected. The light(i.e. the wavelength of the intensity of the light) transmitted throughthe core of the fiber is a function of the change in optical propertiescaused by the interaction of the composition included in the permeablecoating with the material to be detected. The change in opticalproperties may comprise (for example) a change in the index ofrefraction (or indices of refraction differential), or an increase ordecrease in the optical absorbance of fluorescence of the composition.

The above-noted patent discloses materials, compositions, sensingparameters and examples of such sensors, all of which are useful inaccordance with the principles of this invention. In addition,distributed fiber optic sensors for physical properties, such astemperature or pressure, can also be fabricated based on similarproperties, e.g., temperature-induced or pressure-induced changes in therefractive index, optical absorbance, or fluorescence of a cladding orcoating material applied to a light-guiding core.

Unfortunately, such optical fibers are characterized by spatialtransients for transmitted light which causes the sensitivity ofchemically or physically sensitive fibers to vary from point to pointalong the fiber. This is particularly true of multi-mode fibers withlossy (e.g. absorber-doped) coatings where light does not reachequilibrium for a considerable distance. Such a varying response is dueto a spatial transient, associated with the existence of radiationmodes, and strong attenuation of higher order bound modes. Accordingly,sensor response over the length of the fiber is not constant.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with the principles of the invention, the fraction oflight lost per unit length may be controlled by controlling the physicaldesign of the fiber. One use for this invention is to compensate for thespatial transient in distributed fiber optic sensors, wherein a constantsensitivity over the length of a distributed fiber sensor is achieved byintroducing a change in the light-guiding characteristics of the fiberto compensate for the effects of spatial transients. Thus, for example,in one embodiment, a loss-compensated distributed optical fiber sensorcomprises a core and a cladding where the core to cladding refractiveindex differential increases with length to compensate for the loss ofhigher order modes in the (multi-mode) fiber. As a result, a highlyuniform, distributed sensor is realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an optical fiber.

FIG. 2 is a graph of power transmitted vs. fiber position for prior artapparatus.

FIG. 3 is an illustrative embodiment in accordance with the principlesof the invention.

FIG. 4 is a graph of refractive index variation with distance from thelaunch end of a fiber in accordance with the principles of thisinvention.

FIG. 5 is a plot of power transmitted vs. fiber position that resultswhen light is launched into a fiber the refractive index of which variesas shown in FIG. 4.

FIG. 6 is a block diagram of a system using the fiber of FIGS. 1 and 3.

FIG. 7 is a block diagram of a further system using the fiber of FIGS. 1and 3.

FIG. 8 is a plot of energy loss vs. fiber position that results whenlight is launched into the fiber of FIG. 5 when a section of the fiberis exposed to a target chemical substance.

FIG. 9 is a plot of chemical concentration vs. fiber position that canbe derived from FIG. 8.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMNETS OF THE INVENTION

Multi-mode absorption based distributed sensors have an output responsegiven by

$\begin{matrix}{P \approx {P_{o}\frac{V}{\alpha\; l}}} & (1)\end{matrix}$as is well known. In the equation, V is the fiber number given by

$\begin{matrix}{V = {{ka}\sqrt{n_{co}^{2} - n_{cla}^{2}}}} & (2)\end{matrix}$where P_(o) is the power at the input end of the fiber, P is the powerat a distance 1 from the input end, a is the core radius, α is theabsorption coefficient of the cladding, and n_(co) and n_(cla) are coreand cladding refractive indices. Equation (1) is the generic response,which can be observed as a change in total transmitted light intensityusing a simple detection scheme, observed as a plot of backscatteredpower versus length in an optical time domain reflectometer (OTDR), of amulti-mode optical fiber with an absorbing cladding. It is clear thatthe power lost per unit length at each position along a distributedoptical fiber sensor is different from the power lost per unit length atany other position along the fiber.

FIG. 1 shows a representative distributed optical fiber sensor. Thepower can be calculated for each of the positions from the input (di) tothe output (do) and at any of the positions d2, d3, d4, d5, d6 . . . inbetween. Accordingly, the power along the length of the fiber is givenby:

$\begin{matrix}{P = {P_{o}{\exp( {- {\sum\limits_{i - 1}^{N}\;{\int_{0}^{l}{{\alpha(z)}{\eta_{i}(z)}\ {\mathbb{d}z}}}}} )}}} & (3)\end{matrix}$

In accordance with the present invention, a distributed, multi-modeoptical fiber sensor can be made to exhibit a linear response in the dBscale. A linear response to changes in absorption may be achieved bycompensating for the absorption fall off of FIG. 2 by:

a) Increasing the core/cladding refractive index ratio along the fiberlength

b) Increasing the core diameter along the fiber length

c) Increasing the absorption coefficient of the sensor material in thecladding along the fiber length

d) Increasing the absorption coefficient of the cladding along the fiberlength

e) Increasing the scattering coefficient of the core along the fiberlength

f) Varying the parameters (a, b & c) together or in varying combinations

FIG. 2 is a plot of power radiated per unit length versus fiber length lfor a typical multimode fiber. The absorption clearly falls off. Theabsorption coefficient change is due to the fact that all modes in anoptical fiber are attenuated differently. Modes closer to cut off (i.e.,modes that have an angle of incidence in the core/cladding boundarycloser to 90 degrees) are attenuated quicker than modes far from cutoff. In order to obtain an even attenuation as a function of length,coupling from lower order modes to higher order modes must occur. Evenattenuation is achieved, for example, by increasing the core diameter asshown in FIG. 3 or by varying the core/cladding refractive index ratioor by increasing the absorption coefficient or scattering coefficientalong the length of the fiber to compensate for the lower attenuationrate of modes that are increasingly far from cut off.

FIG. 4 is a plot of n_(co)/n_(cla) versus fiber length for a multi-mode,distributed sensor optical fiber in accordance with the principles ofthis invention. It is clear that an optical fiber so designed exhibits apower loss which is linear over the length of the fiber, for thecombined effect of the spatial transient of FIG. 2 and thelength-variation of fiber physical parameters (e.g., the refractiveindex, as shown in FIG. 4) is to linearize the power versus length curve(FIG. 5), thus creating a fiber sensor the sensitivity of which does notvary as a function of length.

FIG. 6 is a block diagram of sensor apparatus employing a fiber opticsensor of the type shown in FIG. 4. The fiber 100 includes a core 125transmissive of electromagnetic radiation (e.g. light) and a cladding orsheath 126 of a permeable material. The index of refraction of sheath126 is lower than that of core 125, and may vary with length. The fibermay include a coating 127 which also would be of a permeable material.

A reactant to be detected, causes a change of, for example, theintensity of light back scattered toward the end into which light islaunched. The input light is launched, and the output light is detectedby detector 131 as noted herein before.

Light energy is introduced at the input end of the fiber by,illustratively, a laser 132 and the properties of the light energy (e.g.intensity, wavelength . . .) are modified by interaction betweenmaterial in the fiber sheath and the reactant to generate backscatteredlight characteristic of the reactant and the interaction with the sheathmaterial.

The importance of structuring the fiber so that this interaction isconstant over the length of the fiber is most clear in applicationswhere the presence of a test chemical (reactant) may be at a point alongthe fiber and a cumulative effect over the length of the fiber is not anissue. The use of an ODTR, as shown in FIG. 7, permits the position ofthe reactant along the fiber to be determined as indicated in FIGS. 8and 9. In addition, such a loss compensated sensor fiber can be used toaccurately measure average chemical concentration over the entire fiberlength by measuring the total light loss induced by the presence of thechemical using the system of FIG. 6. This is not possible for fibers thepower versus length curve of which is nonlinear (e.g. FIG. 2). Anoptical fiber structured in accordance with this invention to exhibit alength-invariant response to such interactions is also advantageous forillumination applications and for applications in which an accumulatedresponse is desired as in U.S. Pat. No. 4,321,057 issued Mar. 23,1982and in the above noted U.S. Pat. No. 4,834,496 where the portion of thelength of fiber affected may be compared to some threshold fortriggering an alarm.

An optical fiber having a core of F-2 Schott glass with a diameter of100 micrometers and a cladding of polymer with a thickness of 20micrometers was fabricated and a twenty meter length of the fiber wastested at 850 nm (active wavelength) as a moisture sensor. A referencewavelength of 1300 nm also was used. A dry reference measurement wasmade and then increasing lengths of 10 cm, 20 cm, and 50 cm were wetted.A final reading was taken after allowing the 50 cm wet length to air dryfor one hour. At 1300 nm only a slight change (<1 dB) in outputintensity occurs after an input signal propagates more than 10 metersalong the fiber. A significant change of 2.4 dB is detected at 850 nm.The OTDR was able to determine the location of the moisture site towithin 1 cm using 850 nm light and 1300 nm light was capable of beingused to compensate for transmission changes due to effects other thanmoisture.

A PH-sensitive fiber was also tested, in a manner similar to that forthe moisture-sensitive fiber, immersed in PH-7 and PH-10 buffersolutions. At 1300 NM there was virtually no change in output intensity.At 850 nm a substantial change of 1.2 dB occurred. The OTDR was able tolocate the position of the buffer solution along the fiber to within 2cms at 850 nm, and again the 1300 nm signal could be used as a“reference”.

The ability to structure a fiber to control the fraction of light lostper unit length in accordance with the principles of this invention alsopermits a fiber to be constructed to provide an arbitrary power lossprofile along the fiber. By so doing, a fiber sensor may be providedwhich is particularly sensitive at a predesignated position in thefiber.

The foregoing Detailed Description of exemplary and preferredembodiments is presented for purposes of illustration and disclosure inaccordance with the requirements of the law. It is not intended to beexhaustive nor to limit the invention to the precise form(s) described,but only to enable others skilled in the art to understand how theinvention may be suited for a particular use or implementation. Thepossibility of modifications and variations within the scope of theclaims will be apparent to practitioners skilled in the art. Nolimitation is intended by the description of exemplary embodiments whichmay have included tolerances, feature dimensions, specific operatingconditions, engineering specifications, or the like, and which may varybetween implementations or with changes to the state of the art, and nolimitation should be implied therefrom. This disclosure has been madewith respect to the current state of the art, but also contemplatesadvancements and that adaptations in the future may take intoconsideration of those advancements, namely in accordance with the thencurrent state of the art. It is intended that the scope of the inventionbe defined by the Claims as written and equivalents as applicable.Reference to a claim element in the singular is not intended to mean“one and only one” unless explicitly so stated. Moreover, no element,component, nor method or process step in this disclosure is intended tobe dedicated to the public regardless of whether or not the element,component, or step is explicitly recited in the Claims. No claim elementherein is to be construed under the provisions of 35 U.S.C. Sec. 112,sixth paragraph, unless the element is expressly recited using thephrase “means for . . . ” and in the absence of recitation of structureand no method or process step herein is to be construed under thoseprovisions unless the step, or steps, are expressly recited using thephrase “step(s) for . . . ”

1. A method to maintain a uniform level of sensitivity in a distributedfiber optic sensor, said sensor comprising a multi-mode single coreoptical fiber having a core and sheath, said method comprising the stepof introducing a change in light-guiding characteristics of said opticalfiber to compensate for effects of spatial transients by varying atleast one parameter of said optical fiber from an input end of saidoptical fiber to an output end thereof.
 2. A method according to claim 1wherein said at least one parameter comprises an increase in thediameter of said core from said input end to said output end.
 3. Amethod according to claim 1 said core having a scattering coefficientand wherein said varying at least one parameter comprises an increase inthe scattering coefficient of said core along the length of said fiber.4. A method according to claim 1 wherein said core is fabricated in amanner to be sensitive to a target chemical.
 5. A method according toclaim 1 wherein said core is fabricated in a manner to be sensitive to aphysical quantity.
 6. A method according to claim 1 wherein said sheathincludes a cladding and said cladding is fabricated in a manner to besensitive to a physical quantity.
 7. A method according to claim 1wherein said sheath includes a cladding and said cladding is fabricatedin a manner to be sensitive to a target chemical.
 8. A method accordingto claim 7 said cladding having an absorption coefficient and whereinsaid at least one parameter comprises an increase in the absorptioncoefficient of said cladding along the length of said fiber.
 9. A methodaccording to claim 7 said optical fiber having a core/claddingrefractive index ratio and wherein said at least one parameter comprisesan increase in the core/cladding refractive index ratio along the lengthof said fiber.
 10. A method according to claim 1 wherein said sensor isa distributed chemical agent sensor.